The purpose of a vehicular transmission is to provide a neutral, at least one reverse and one or more forward driving ranges that impart power from an engine, and/or other power sources, to the driving wheels, as required to provide the necessary driving force and the desired performance.
A series propulsion system is a system in which energy follows a path from an engine to an electric storage device and then to an electrical motor which applies power to rotate the wheels. There is no direct mechanical connection between the engine and the wheels in a series propulsion system.
Transmissions adapted to receive the output power from either an engine or an electric motor, or both, have heretofore relied largely on what has been designated as series, hybrid propulsion systems. Such systems are designed with auxiliary power units (APU's) of relatively low power for minimum emissions and best fuel economy. However, such combinations of small APU's and even large energy storage devices do not accommodate high-average power vehicles or address duty cycles that demand continuous, constant speed operation. Steep grades and sustained high-average cruising speeds at desired high efficiencies are not achievable with a typical, series, hybrid transmission configuration.
The challenge, therefore, is to provide a power system that will operate at high efficiencies over a wide variety of operating conditions. Desirable electric variable transmissions should leverage the benefits of a series, hybrid transmission for desirable low-average power duty cycles--i.e.: low speed start/stop duty cycles--as well as the benefits of a parallel hybrid transmission for high-average output power, high speed duty cycles.
Moreover, perfecting a concept wherein two modes, or gear trains, are available for synchronous selection by the on-board computer to transmit power from the engine and/or the motor/generator to the output shaft results in a hybrid transmission having an extremely wide range of applications.
One of the most successful substitutes for the series hybrid transmission is the variable, two-mode, input-split, parallel, hybrid electric transmission. Such a transmission utilizes an input means to receive power from the vehicle engine and a power output means to deliver power to drive the vehicle. First and second motor/generators are connected to energy storage devices, such as batteries, so that the energy storage devices can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage devices and the motor/generators as well as between the first and second motor/generators.
A variable, two-mode, input-split, parallel, hybrid electric transmission also employs at least one planetary gear set. The planetary gear set has an inner gear member and an outer gear member, each of which meshingly engages a plurality of planet gear members. The input means is operatively connected to one of the gear members in the planetary gear set, and means are provided operatively to connect the power output means to another of the gear members in the planetary gear set. One of the motor/generators is connected to the remaining gear member in the planetary gear set, and means are provided operatively to connect the other motor/generator to the output shaft.
Operation in the first or second mode may be selectively achieved by using torque transfer devices. In one mode, the output speed of the transmission is proportional to the speed of one motor/generator, and in the second mode the output speed of the transmission is generally proportional to the speed of the other motor/generator.
In some embodiments of the variable, two-mode, input-split, parallel, hybrid electric transmission a second planetary gear set is employed. In addition, some embodiments may utilize three torque transfer devices--two to select the operational mode desired of the transmission and the third selectively to disconnect the transmission from the engine. In other embodiments, all three torque transfers may be utilized to select the desired operational mode of the transmission.
With reference, again, to a planetary gear set, the planetary gear members are normally supported for rotation on a carder that is itself rotatable. When the sun gear is held stationary and power is applied to the ring gear, the planet gear members rotate in response to the power applied to the ring gear and thus "walk" circumferentially about the fixed sun gear to effect rotation of the carrier in the same direction as the direction in which the ring gear is being rotated.
When any two members of a planetary gear set rotate in the same direction and at the same speed, the third member is forced to turn at the same speed, and in the same direction. For example, when the sun gear and the ring gear rotate in the same direction, and at the same speed, the planet gears do not rotate about their own axes but rather act as wedges to lock the entire unit together to effect what is known as direct drive. That is, the carrier rotates with the sun and ring gears.
However, when the two gear members rotate in the same direction, but at different speeds, the direction in which the third gear member rotates may often be determined simply by visual analysis, but in many situations the direction will not be obvious and can only be determined by knowing the number of teeth present in the gear members of the planetary gear set.
Whenever the carrier is restrained from spinning freely, and power is applied to either the sun gear or the ring gear, the planet gear members act as idlers. In that way, the driven member is rotated in the opposite direction as the drive member. Thus, in many transmission arrangements when the reverse drive range is selected, a torque transfer device serving as a brake is actuated frictionally to engage the carrier and thereby restrain it against rotation so that power applied to the sun gear will turn the ring gear in the opposite direction. Thus, if the ring gear is operatively connected to the drive wheels of a vehicle, such an arrangement is capable of reversing the rotational direction of the drive wheels, and thereby reversing the direction of the vehicle itself.
As those skilled in the art will appreciate, a transmission system using a power split arrangement will receive power from two sources. Utilization of one or more planetary gear sets permits two or more gear trains, or modes, by which to deliver power from the input shaft of the transmission to the output shaft thereof. As such, it is well known in the an that a multi-range, power split, hydro-mechanical, or hydrostatic, transmission will utilize at least one planetary gear set. Typically, a planetary gear set will have one member connected to the power source, one member connected to the output of the transmission, and the final member of the planetary gear set will be connected to a hydrostatic drive. This is particularly advantageous because a hydrostatic drive has the same speed and torque capabilities in the reverse direction as it does in the forward direction, which is useful in military type vehicles, such as tanks and personnel carriers.
However, a transmission for a passenger vehicle, such as a bus, has somewhat different requirements. For example, a bus only requires that the maximum reverse speed be twenty percent (20%) of the maximum forward speed capacity, while still requiring that the maximum power available for the forward drive range also be available for reverse drive range.
By utilizing a hydrostatic unit in combination with a planetary gear system, an efficient, multi-range power transfer system can be developed for a passenger vehicle. The planetary gear system is very useful in combining two input power sources, such as the mechanical input received directly from an engine and the input power received from the hydrostatic drive. The efficiency of this configuration is dependent upon how much power is transferred through the hydrostatic drive. The more power that is received directly from the engine (the mechanical path), the higher the overall efficiency of the transmission. Thus, the maximum efficiency of the transmission is realized when the speed of the hydrostatic drive is zero.
As is well known to the art, there are several drawbacks in the use of hydrostatic drives in transmission systems. Typically, hydrostatic pumps and motors are not conducive to concentric and compact transmission design. For example, efficient hydrostatic units cannot accept another shaft operating concentrically through their pump and motor shafts. Therefore, a hydrostatic pump and motor will not be on the transmission centerline, but on a parallel main shaft with external gearing to transfer power from the hydrostatic unit to the centerline of the transmission. As a result, a hydrostatic pump and motor are not compatible with compact concentric transmission designs. Furthermore, hydrostatic units that operate at greater than 5,000 pounds per square inch (psi) are very noisy.
One possible solution to the aforementioned problems, is to replace the hydro-mechanical transmission with an all-electric drive transmission. However, all-electric transmission units have their own peculiar problems. In particular, the all-electric units are inefficient because engine power is transformed into electrical power to drive a motor, and that electrical power is then transformed back into mechanical, rotary power by the motor to drive the vehicle. Thus, each transformation of mechanical to electrical power, and vice versa between the engine and the drive wastes power.
As successful as the variable, two-mode, input-split, parallel, hybrid electric transmission is, no mechanical point exists in the first mode. That is, neither of the motor/generators is stationary at any time during operation of the transmission in the first mode. This is a drawback inasmuch as the maximum mechanical efficiency in the transfer of power from the engine to the output occurs when one of the motor/generators is at a mechanical point--i.e.: stationary. In variable, two-mode, input-split, parallel, hybrid electric transmissions, however, there is one point in the second mode at which one of the motor/generators is not rotating such that all the engine power is transferred mechanically to the output.
Accordingly, there is a need in the art for a large, horsepower transmission system which provides maximum power with little additional power provided by the electric storage device. It is also desirable to enhance overall efficiency at high output speeds.